Perhaps this one isn’t strictly Czech, but I saw it done here first. Instead of cutting a dumpling with a knife and thereby squishing it, take a length of string, ordinary cotton does the trick, pull it under the dumpling and then cross the ends over and pull the string through. You should get a nicely formed dumpling slice. String can also be use to cut cakes, especially the more delicately constructed ones, and soft cheeses, though it’s enough to pull the string taut and push down through the food.